1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a light guide panel in which luminous efficiency is improved using polarization recycling, and a back light unit having the same.
2. Description of the Related Art
Traditionally, back light units are used as light sources for displays, such as liquid crystal displays (LCDs), that are not self-luminescent but rather form an image using incident light from an external source.
Back light units can be classified as direct light emitting type back light units or as edge light emitting type back light units, depending on whether a light guide panel (LGP) is used and on the arrangement of a light source.
In direct light emitting type back light units, a lamp installed opposite a surface of an LCD directly irradiates light onto a liquid crystal panel. On the other hand, in edge light emitting type back light units, an LGP is disposed opposite the liquid crystal panel, and a light source is disposed at an edge of the LGP so that light irradiated from the light source is guided by the LGP to be transmitted to the liquid crystal panel.
A liquid crystal panel selectively transmits incident light according to the polarization direction of the light, and only that light which is polarized in a predetermined direction is utilized.
FIG. 1 is a perspective view of a conventional LGP. The LGP 1 has a flat panel type structure with a rectangular parallelepiped. The LGP 1 includes an incident light surface 3 on which light L is incident, an opposite surface 5 formed on a surface opposite to the incident light surface 3, a reflection surface 7 which reflects the light L incident through the incident light source 3 toward the opposite surface 5, and an emitted light surface 9, which is opposite the reflection surface 7 and from which the light is emitted.
The conventional LGP 1 having the above structure has a reflection surface 7 with a planar structure. Thus, a polarization direction of incident light is not changed by the reflection surface 7. Thus, the polarization direction of light incident on an LCD panel and the polarization direction of light reflected from the reflection surface 7 are identical to each other.
A change of polarization direction will now be described with reference to FIGS. 2A and 2B and Tables 1 and 2.
FIG. 2A illustrates polarization directions of light incident through the incident surface 3 when ten light beams are incident on the LGP 1 illustrated in FIG. 1 at an incident angle θi=60°. Table 1 shows values of polarization of light L on the X-, Y-, and Z-axes.
TABLE 1LightXYZ10.00000000.94217520.335120620.00000000.94217520.335120630.00000000.94217520.335120640.00000000.94217520.335120650.00000000.94217520.335120660.00000000.94217520.335120670.00000000.94217520.335120680.00000000.94217520.335120690.00000000.94217520.3351206100.00000000.94217520.3351206
FIG. 2A and Table 1 illustrate that light is linearly polarized in an Y-Z direction.
FIG. 2B illustrates polarization directions of light reflected from the reflection surface 7 when ten beams of light are incident on the LGP 1 illustrated in FIG. 1 at an incident angle θi=60°. Table 2 shows polarization values of light L on the X-, Y-, and Z-axes.
TABLE 2LightXYZ10.00000000.94217520.335120620.00000000.94217520.335120630.00000000.94217520.335120640.00000000.94217520.335120650.00000000.94217520.335120660.00000000.94217520.335120670.00000000.94217520.335120680.00000000.94217520.335120690.00000000.94217520.3351206100.00000000.94217520.3351206
FIG. 2B and Table 2 illustrate that light is linearly polarized in the Y-Z direction. The polarization direction and values of reflected light is identical to that of the incident light described with reference to FIG. 2A and Table 1.
Thus, when unpolarized light is incident on the LGP 1, light emitted through the emitted light surface 9 also has the same polarization characteristics as the incident light and thus is also unpolarized light. Therefore, when the LGP 1 is used for illumination of an LCD screen, which utilizes only light polarized in a predetermined direction, luminous efficiency is lowered.